Apparently the specification should have specified signed integers for the cHRM fields: pnggroup/libpng#579 (comment)

• Remove range checks

• Use signed ints when converting to/from doubles with spng_chrm

Should probably add a note for bindings to pretend the fields for spng_chrm_int are signed, the types for the raw values will not be changed for a while.

Note: waiting for a final change in the PNG specification